This invention relates generally to a method of preparing a redox battery electrolyte containing chromium ion and/or iron ion.
There is an increasing demand for an electric power storage system and redox batteries are now attractive for such an energy storage. One known redox-flow type battery includes positive and negative electrodes disposed respectively in positive and negative electrode compartments separated by an ion exchange membrane. An electrolyte is contained in each compartment and recirculated by a pump. When, for example, iron ion-containing aqueous hydrochloric acid solution and chromium ion-containing aqueous hydrochloric acid solution are used as positive and negative side electrolytes, respectively, the following redox reactions occur: ##STR1##
During nighttime, surplus electric power is subjected to AC--DC conversion by an inverter and the resulting direct current is used to cause the above reactions to proceed in the charge directions, so that the electric power is stored in the battery. During daytime, on the other hand, the above reactions are allowed to proceed in the discharge directions and the discharged electric energy is supplied to a power supplying system after being subjected to DC--AC conversion in the inverter.
In such a redox battery, it is important that the content of impurity metal content in the electrolytes should be minimized in order to suppress the occurrence of side reactions in the negative electrode which result in the formation of hydrogen gas and which adversely affects the coulomb efficiency of the battery. While the use of a high purity raw material such as high grade metallic iron, high grade metallic chromium or high grade chromic acid can suppress such hydrogen gas forming reactions, they are too expensive to be used in redox batteries on a commercial scale.
Japanese published unexamined patent application (Tokkyo Kokai) No. 60-115,174 suggests a method of preparing an electrolyte for use in redox flow batteries from an inexpensive raw material such as chrome minerals, chromite or ferrochrome. This method includes the steps of dissolving the raw material in an acid solution containing hydrogen chloride, introducing the resulting solution into a cathode compartment of an electrolytic cell, and subjecting the solution in the compartment to electrolysis to cause heavy metals contained therein to deposit on the electrode. In the specific working example, the electrolysis is performed with an electric potential of +0.05 V vs saturated calomel electrode (hereinafter referred to as SCE for brevity).
An electrolyte obtained by the prior art method, however, has been found still to cause lowering of the coulomb efficiency of the redox battery upon repeated use.